How will you explain the fact that first ionisation enthalpy of sodium is lower than that of magnesium but its second ionisation enthalpy is higher than that of magnesium?

The first electron in both the cases has to "be removed from 3s-orbtials, but nuclear charge of Na is less than that of Mg. Hence I.E. of Na is lower them that of Mg.

After the loss of first electron the electronic con-figuration of {{Na}^{+}} is1${{s}^{2}}2{{s}^{2}}2{{p}^{6}}, i.e., that of noble gas which is very stable and hence the removal of 2nd electron from {{Na}^{+}} is very difficult. In the case of Mg, after the loss of first electron, electronic configuration of {{Mg}^{+}} ions is 1{{s}^{2}}2{{s}^{2}}2{{p}^{6}},3{{s}^{1}}. The second electron to be removed is from 3s orbital which is easier. Hence I{{E}{2}} of sodium is much larger thanI{{E}{2}}$ of Mg.